substr

Extracts a substring out of EXPR and returns it. First character is at
offset zero. If OFFSET is negative, starts
that far back from the end of the string. If LENGTH is omitted, returns
everything through the end of the string. If LENGTH is negative, leaves that
many characters off the end of the string.

You can use the substr
function as an lvalue, in which case EXPR
must itself be an lvalue. If you assign something shorter than LENGTH,
the string will shrink, and if you assign something longer than LENGTH,
the string will grow to accommodate it. To keep the string the same
length, you may need to pad or chop your value using
sprintf.

If OFFSET and LENGTH specify a substring that is partly outside the
string, only the part within the string is returned. If the substring
is beyond either end of the string,
substr returns the undefined
value and produces a warning. When used as an lvalue, specifying a
substring that is entirely outside the string raises an exception.
Here's an example showing the behavior for boundary cases:

An alternative to using
substr as an lvalue is to
specify the
replacement string as the 4th argument. This allows you to replace
parts of the EXPR and return what was there before in one operation,
just as you can with
splice.

Note that the lvalue returned by the three-argument version of
substr acts as
a 'magic bullet'; each time it is assigned to, it remembers which part
of the original string is being modified; for example: